MPEG4Extractor: reduce buffer size for audio tracks

We don't need 3.4 MB buffers for audio, and also don't need 8 of them.
Reduce the size of audio buffers, and allocate fewer buffers initially,
but leave the limit at 8.

Bug: 79420581
Test: manual, Camera, CTS
Change-Id: I180c42c59735b9f04eeeea4f4bd5976da50f5ec6